On a university campus just outside Boston, on a summer's day six years ago, the Argentine players were getting ready for the World Cup by playing a scratch game among themselves. So good was the mood that Diego Maradona stopped giving interviews and joined in. Every goal at either end was cheered by hundreds of rich Argentines studying in Boston.

Yet it was quiet enough to hear the players shouting. And the word they shouted most often was 'Flaco!'

None of the Argentine players was named Flaco, but in Latin America you only become a real person once you acquire a nickname, and 'Flaco' - 'Thin One' - was the sobriquet of Fernando Redondo.

The reason everyone kept screaming it was that Redondo always seemed to have the ball. In this brilliant team, which but for a cocktail of ephedrine would have won the World Cup, all attacks ran through the long-haired, hippie-like figure at the back of midfield. So they do at Real Madrid, the team that on Wednesday night plays Valencia for the Champions League.

Real's chances depend largely on El Flaco (or The Prince, or The White Machine). At his best he can destroy Manchester United. At his worst, he infuriates Johan Cruyff. And whether Redondo will be at his best depends largely on his team. Real Madrid have just overcome United and Bayern Munich, but a year ago, with much the same team, in the Spanish Cup semi-finals, they lost 6-0 to Valencia. 'San Marino, you are like San Marino,' sang the Valencia fans. So which Flaco and which Real will we get?

Redondo was born 30 years ago in Adrogué in Buenos Aires. It is common to say of any Latin American footballer that the game helped him escape from the slums. This is not true of Redondo. A middle-class boy, he was studying law when Carlos Bilardo, then the Argentine manager, called him up for some friendlies before the World Cup of 1990.

Redondo turned him down, saying he had to sit exams. The truth is more complex. There are two basic types of Argentine coaches: those such as Bilardo and Daniel Passarella, who think football is about breaking legs, diving for penalties and punching goals, and those such as Jorge Valdano, Cesar Luis Menotti and the current Argentine manager, Marcelo Bielsa, who think there is more to it. The latter tend to be educated men who compare football with poetry, music and countless other art forms.

The Bilardistas are generally on the political right: Passarella captained the Argentine side that won the 1978 World Cup under the military regime. The Menottistas are mostly left-wingers.

Redondo, an educated man of the left, is with the Menottistas. This is more than a matter of academic interest: it has shaped his career, and explains why he is at Real. After rejecting Bilardo, he moved to Europe to play for Tenerife, where his manager was Valdano. It was Valdano who brought him to Real Madrid in 1994, and it was Bielsa who recalled him to the Argentine side.

Before the last World Cup Passarella had kicked Redondo out of the Argentine team. Famously, Redondo had ignored Passarella's edict to cut his hair. In fact, the hair was merely the excuse. The dispute went deeper: Passarella wanted Redondo to consider himself a soldier in his army. Redondo would not.

Passarella ended up looking silly. Argentina went out of the World Cup in the quarter-finals, weeks after Redondo had helped Real Madrid win the Champions League.

At Real, Redondo has won everything: two league titles and the World Club Cup, as well as the 'septima', the club's seventh European Cup. Yet it would be wrong to say that he has had six glorious years. Real are a crazy club, led by a president with a three-second attention-span, and Redondo has often been dragged down with the rest.

This year Real have climbed out of the bottom half of the Spanish league but never got a whiff of the title. Their home defat on Friday means they have finished fifth and must win on Wednesday to return to next season's Champions League. 'Real Madrid have been playing fairly badly to very badly all year,' said Johan Cruyff in March. 'I don't think there is a team in the world where the midfield loses so many balls. You can't count how many Redondo loses.'

Redondo is often called the master of the one-touch. 'Tactically perfect,' said his former coach Fabio Capello. But when his team-mates are not moving, he can appear helpless. He lacks acceleration, is often carrying injuries, and sometimes seems not to care. Then he becomes the master of the two or three-touch, passing needlessly to defenders, slowing the game down.

Real have continued to falter, but this spring they have at least played well in the big games. A month ago they saved their season by beating Manchester United 3-2 at Old Trafford, chiefly thanks to Redondo. Playing further forward than usual, he also produced the moment of this year's Champions League.

First, it must be said that humiliating United's Henning Berg is a realistic proposition. It has been done before, usually by Berg himself. But Redondo defined the art. Sprinting with Berg to the goalline, he back-heeled the ball through the Norwegian's legs, collected it and passed to Raúl who scored Real's third goal. Later Rinus Michels, the authoritarian former manager of Holland, was asked about that back-heel. Surely he would never have tolerated it? 'That boy,' said Michels, 'would never have heard anything about it from me.'

Alex Ferguson noted that all loose balls that night had seemed to fall to Redondo. 'What does this player have in his boots?' Ferguson asked. 'A magnet?' Redondo is indeed more than a playmaker - at his best, he hoovers the midfield.

United escaped lightly. In the semi-finals Redondo and Raúl deliberately humiliated Bayern Munich. At times the two had fun simply passing to each other, making jokes about Carsten Jancker as the huge striker lumbered after the ball. More than once Redondo showed the ball to Bayern's Stefan Effenberg, waited for Effenberg to pounce, and then rolled it to a team-mate.

The Germans also sulked about a particularly frivolous bicycle-kick by Roberto Carlos. This, they felt, was bad manners.

In this Real side, Redondo and Raúl - perhaps the two key players - are best mates. Both like their peace before games. In the next few days, while his colleague Steve McManaman whiles away the time watching films and Nicolas Anelka and Geremi are busy on their Playstations, Redondo and Raúl will be reading bound volumes of paper known as books. The Thin One enjoys Jorge Luis Borges, Gabriel García Márquez and Italian fashion magazines.

Wednesday could be Redondo's day: Anelka, Raúl and Fernando Morientes are exactly the quick forwards he needs. Alternatively, Valencia could win 6-0 again.